What Causes Thickened Endometrium While Having Polyp?

Hi. My mom has just had a biopsy of her endometrium done as an ultrasound shows it to be 16mm thick. She is 70 and is postmenopausal. She had been bleeding but the doctor found a polyp (which was bleeding) during the biopsy and removed it. We are hoping that was causing the problem. What are the chances of it still being endometrial cancer? Can there ever be any other causes of an endometrium being so thick? She has never been on any HRT. Thank you.